The invention relates to a satellite navigational system and method. More particularly, it relates to a navigational system and method using satellites for transmitting coded time signals as well as additional navigational data used for determining position, locating operations, navigation, etc.
In the navigation of vehicles, proper position and speed must be determined within a given reference system. The reference points conventionally used are radio beacons in a precisely known, fixed position. With the increasing accuracy in the determination of satellite positions, the use of satellites as reference points for high-precision locating and navigating operations has become possible. The use of satellites brings about substantial advantages. Due to the higher frequencies involved, measuring accuracy is enhanced. Longer reaching and more economical systems may be developed as satellite coverage becomes available over large areas on the earth's surface. The three-dimensional reference points provided by satellites permit three-dimensional and, thus, more precise fixing of vehicle positions, satisfying most of the user's requirements.
However, as compared to terrestrial radio beacons, the use of satellites requires additional expenditures as the positions of satellites are subject to constant change, and operation as well as synchronization of the high-stability time signal transmitters within these satellites requires specific techniques. Thus, any receiver of ranging signals from a satellite must be continuously supplied with other information as well, i.e., data on the position of the respective satellite and on the exact reading of the satellite clock at the time of transmission of the signals, so as to be able to determine its proper position from the signals received.
At present, irrespective of other existing navigational systems, satellites positioned in geosynchronous orbit above the equator of the earth are being used for establishing communication between cruising ships and control stations on the shore. For the transmission of distress signals, experiments are being conducted with satellites either orbiting across the polar regions or positioned in geosynchronous orbit above the equator. Also proposed was the distribution of information under simultaneous interrogation of vehicle positions by means of geostationary satellites.
Satellite navigation in the specific sense means determining the proper position and, if necessary, the proper movements of a vehicle with the aid of satellites. As in terrestrial radio locating systems, proper position is determined by the propagation time and/or change in propagation time of electromagnetic waves between the vehicle and a number of reference points. Two or three-dimensional position fixing requires measuring operations involving from two to four reference points depending on whether the single-way or two-way method is used. For high measuring accuracy, these reference points must also be in a favorable geometrical position relative to the cruising vehicle. As such, favorable geometrical groupings covering the entire surface of the earth cannot satisfactorily be realized using satellites in geosynchronous orbit. A navigational satellite system extending around the earth always includes orbiting satellites in a non-geosynchronous position, with their orbital paths at a lower plane than those of geosynchronous satellites.
The best known and hitherto only operational navigational satellite system of this type is the so-called TRANSIT system. In this system, each space segment includes five satellites positioned in polar orbits transmitting time signals on two different frequencies, together with data on their respective orbits. Slow moving vehicles may determine their positions at larger time intervals by measuring and utilizing, respectively, the Doppler effect during each transit recurring at intervals of from two to ten hours. However, to have a sufficient number of satellites available for position fixing at any time at each location on the earth's surface, a plurality of satellites must continuously circle the earth at varying planes of orbit (three-dimensional single-way position fixing requires e.g., four simultaneously visible satellites). A system presently being set up in the United States of America, the so-called NAVSTAR-GPS (Global Positioning System), which will be put in operation in 1987 or 1988, was to comprise twenty-four of such navigational satellites; to save costs, the number of satellites was recently reduced to eighteen. The alternative European systems NAVSAT and GRANAS now in the planning stage will require about the same number of navigational satellites. However, the intent is to lower the total expenses for the two European systems below those of the U.S. system by spending less for the individual satellites and tolerating somewhat higher expenditures for the ground equipment. Thus, the navigational satellites proposed for both European systems are not to include the high-stability oscillators which are a major factor in the cost of the U.S. navigational system NAVSTAR-GPS.
The synchronized time signals required for single-way ranging operations shall instead be produced on the ground, together with the above-mentioned additional data. They will be transmitted from a plurality of ground stations distributed over the earth to the navigational satellites and retransmitted from there to the cruising vehicles. Installation and operation of these extensive and complex ground stations will, however, consume a large part of what has been saved in the equipment of the satellites. Not even the proposed modification of the European GRANAS system will noticeably reduce the total costs of the system. The intended simplification of the ground stations through proper position fixing within the satellites will again bring about somewhat higher expenditure for the satellites. Of the other navigational satellite system, GLONAS, presently being installed in the Soviet Union, it is known only that it has a strong resemblance to the U.S. system NAVSTAR-GPS.
All presently existing or planned navigational systems using satellites utilize a plurality of similarly designed, orbiting satellites which emit locating signals, in the form of time marks, as well as additional, satellite specific information, i.e., data required for reading and utilizing these time signals in position fixing and navigational operations. This data is corrected at certain intervals by the ground control equipment and subsequently retransmitted to the satellites for storage. Also included is data for contacting other navigational satellites and conducting propagation corrections, or retransmission of the locating signal on a second carrier frequency. Moreover, all previously disclosed or proposed systems are limited to the determination of proper position and speed of vehicles. The various systems are incompatible with each other and a user must have available a variety of equipment if he wants to receive signals from the navigational satellites of the various systems. Thus, because of the high expenditure required for each satellite serving the above functions, the costs of an international, global system are very high as 18 to 24 of such satellites are needed for an adequate global coverage.